Hummingbirds turned savoury into sweet to taste nectar

It's the strangest sweet tooth in the world. Birds lost the ability to taste sugars, but nectar-feeding hummingbirds re-evolved the capacity by repurposing receptors used to taste savoury food.

To differentiate between tastes, receptors on the surface of taste buds on the tongue, known as T1Rs, bind to molecules in certain foods, triggering a neurological response.

In vertebrates such as humans, a pair of these receptors – T1R2 and T1R3 – work together to deliver the sweet kick we experience from sugar. But Maude Baldwin at Harvard University and her colleagues found that birds don't have the genes that code for T1R2. They are found in lizards, though, suggesting that they were lost at some point during the evolution of birds or the dinosaurs they evolved from.

But hummingbirds clearly can detect sugar: not only do they regularly sup on nectar, taste tests show they prefer sweet tasting foods over blander options. Now Baldwin and her team have worked out why: another pair of receptors – T1R1 and T1R3 – work together to detect sugar.

Savoury to sweet

Other vertebrates use T1R1 to taste savoury foods. It seems that in hummingbirds the proteins on the surface of the two receptors have been modified so that they respond to sugars instead.

"The change in the taste receptor was certainly not the only factor or aspect of hummingbird biology that was important [for them to feed on nectar], but it seems like it played an important role," says Baldwin. "There are many behavioural and physiological changes that have occurred between hummingbirds and their ancestors: small body size, a long bill and changes in the wing which allowed them to hover."

"We know a lot about bird vision and smell, but until recently very little was known about the genetic basis of taste in any bird species," says Hannah Rowland at the University of Cambridge, who was not involved in the study. "These findings should help researchers test sweet perception in other birds that eat fruit and nectar. The question for me is whether other nectar eating birds and frugivores have evolved this same capacity."

The re-evolution of sugar receptors may have happened multiple times, says study member Stephen Liberles, also at Harvard. "It will be exciting to see how other nectar feeding birds taste sugar, to compare whether evolution used the same or different strategies to solve the problem of sugar detection," he says.

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